Recently, among color display devices used as image devices of computers or televisions, a plasma display device has drawn attention as a color display device which can realize a thin, light and large display.
Three primary colors (a red, green and blue) are added by an additive process, so that the plasma display device performs a full color display. For performing the full color display, the plasma display device has phosphor layers for emitting red, green and blue light which are three primary colors. In a discharge cell of the plasma display device, ultraviolet rays having a frequency of 200 nm are generated by discharge of rare gas. Then each phosphor is excited by the ultraviolet ray, whereby each visible light is generated.
As the phosphor for emitting light discussed above, for example, (Y, Gd) BO3:Eu3+, Y2O3:Eu3+ are known for emitting red light. (Ba, Sr, Mg) O.aAl2O3:Mn2+, Zn2SiO4:Mn2+ are known for emitting green light. BaMgAl10O17:Eu2+ is known for emitting blue light.
In the blue phosphor whose parent body is BaMgAl10O17 and called BAM base, Eu of an emission center is required to be activated at divalence to intensify brightness of emission. An example for firing in a reducing atmosphere is disclosed at page 170 of “Phosphor Handbook” edited by Phosphor Research Society and published by Ohmsha, Ltd.
When the phosphor is fired in an oxidizing atmosphere, Eu becomes trivalent and can not correctly replace divalent Ba in the parent body. Therefore, Eu can not be an active emission center, so that brightness of emission decreases. Furthermore, the blue phosphor does not perform its purpose and generates red emission of Eu3+ its own.
Europium activated acid yttrium sulfide (Y2O2S:Eu3+) of a red phosphor is required to be activated at trivalence, so that it is produced by firing in an oxidizing atmosphere. In a phosphor whose host crystal is constituted of an oxide, it is commonly thought that oxygen atoms are removed from the host crystal in firing, so that oxygen deficiencies are generated. As a method for repairing the oxygen deficiencies, an example for firing Y2O2S:Eu3+ where Eu is activated at trivalence by inert gas containing oxygen is disclosed in Japanese Patent Unexamined Publication No. 2000-290649.
However, as compared with a phosphor oxide produced by firing in an oxidizing atmosphere, in a phosphor oxide produced by firing in a reducing atmosphere, an oxide is easy to be removed from the host crystal in the reducing atmosphere, so that oxygen deficiencies in the host crystal increase. In addition, if a phosphor oxide, which has to be fired in a reducing atmosphere, is fired in an oxidizing atmosphere, it is difficult to maintain the original number of valence of an activator.
In other words, when ion impact accompanied by discharge or irradiation of ultraviolet rays is applied to the phosphor whose host crystal has many oxygen deficiencies, the phosphor tends to deteriorate with time, where the ultraviolet rays having high energy and 147 nm frequency are generated at the plasma display device. It is thought that bonding between atoms is weak at a place where oxygen deficiencies exist. Therefore, when ultraviolet rays of high energy or ion impact are applied to such a place, a crystal structure falls into disorder and the place tends to be amorphous. The amorphous place means deterioration of the host crystal, so that deterioration of brightness with time, color shift by chromaticity variation, image burn-in or the like is generated in the plasma display device.
If a phosphor oxide, which has to be fired in a reducing atmosphere, is fired in an oxidizing atmosphere to repair the oxygen deficiencies, for example, Eu becomes trivalent Eu3+ in BAM base phosphor, so that conspicuous deterioration of brightness is generated.
The present invention is directed to solve the problem discussed above, and aims to provide a method of preparing a phosphor which can repair oxygen deficiencies without deterioration of emission brightness and a plasma display device with a phosphor having high emission brightness and slight deterioration of brightness, even in the phosphor whose host crystal is an oxide and Eu or Mn of an emission center is required to be activated at divalence.